This invention relates in general to hydrostatic altimeters, and in particular to those having an elongated, liquid filled, flexible tube linking a body of liquid at one end of the tube and a pressure sensor at the other end of the tube, the difference in hydrostatic pressure at the opposite ends being indicative of the altitude of the sensor end relative to the body of liquid.
Hydrostatic altimeters, such as disclosed in U.S. Pat. No. 4,878,297 by Vories, offer many advantages over conventional optical methods of elevation measurement and leveling. A hydrostatic altimeter works on the basis of pressure developed within a liquid filled tube ("link") due to the density or specific weight of the liquid ("link liquid") and the liquid elevation difference between the link's two ends. This principle of operation permits a single operator to use the hydrostatic altimeter around corners or among brush and trees where optical or laser levels will not function. For a hydrostatic altimeter to be a precision instrument under all environmental conditions, all environmental parameters which influence its accuracy must be corrected by appropriate compensation. Pressure transducer and processing electronics zero offset, span, non-linearity and drift of these and other parameters can be readily compensated for within the device, but density changes in the link liquid caused by changes in temperature must also be compensated for since these changes effect the pressure measured by the transducer at the ends. A ratio of unit changes in the density of a liquid per unit changes in temperature is commonly known as the temperature coefficient of density (TCD) of the liquid. To date, patents and published literature offer no means for adequately compensating for the TCD of a link liquid.
One U.S. patent addresses the TCD of a link liquid. Gaucher et al. in U.S. Pat. No. 4,397,099 describes a two transducer hydrostatic altimeter (two absolute pressure transducers are used to measure pressure at both extreme ends of the tube) having a resistance wire temperature sensor that runs within the entire length of the link tube in order to measure the average temperature of the liquid link. This measurement is used to compensate for temperature effects on the link liquid. However this scheme does not take into account the role of each segment of the link in the overall vertical liquid column, i.e. the role of each segment in developing the hydrostatic pressure. This scheme only works properly if the entire length of the link is used and is uniformly sloped - a condition that seldom exists in reality. A scheme such as this can actually be worse than no compensation at all under most circumstances. For example, if 90% of the link is coiled up in the shade and only 10% of the link is actually used in a sunny area to measure elevation, the Gaucher TCD compensation circuit will falsely correct by 90% in favor of the shade rather than for the 10% of the link at the warmer temperature where the liquid column is actually contributing to the reading. Similarly, dramatic compensation errors can occur when a link lies at various slopes and through traverses through various temperatures.
To avoid these problems it is necessary to integrate a temperature-pressure or density-pressure product along the entire length of the link. In other words, compensation temperature or density should only be measured and averaged in proportion to its vertical column contribution to the total link pressure. Where there is no association with a vertical column or pressure head there should be no temperature or density contribution to the average.
Another problem of equal or greater significance addressed by this invention relates to dissolved gases in the link liquid which can be drawn out of solution or past tubing joints or fittings under negative hydrostatic pressures that can develop during normal operation of an hydrostatic altimeter. According to Henry's Law, all gas solubility in liquids is directly proportional to the pressure at a gas or bubble interface with the liquid and inversely proportional to the temperature of the liquid. Even if dissolved gases were removed from a liquid prior to installation in a sealed hydrostatic altimeter, nearly all flexible tubing is permeable to gases and it is always possible that the sensor or associated joints could leak air into the liquid under negative pressures. An excessive quantity of trapped gases can not be absorbed into a gas saturated liquid and can form bubbles at unpredictable locations along the link, thereby reducing the pressure measured at the transducer and causing erroneous readings. Gases coming out of solution cause bubbles in the liquid which can cause erroneous readings.
Another problem addressed by this invention is the frequent occurrences of differences in atmospheric pressure between the locations of the link ends. Such differences can, for example, occur in buildings due to an unbalance heating, ventilation or air conditioning system. Outdoors they can occur as a result of wind particularly around buildings or obstructions. Similarly, a difference can occur due to a difference in altitude between the link ends, but this has been found to be negligible over the practical vertical span of such altimeters.
This invention can provide liquid temperature compensation, prevent bubble formation, and eliminate problems caused by atmospheric pressure differences. Other advantages and attributes of this invention will be readily discernable upon a reading of the text hereinafter.